The root of the conflict between the United States and Mexico from 1846 to 1848 was the Mexican-American War. The war was fought over the disputed territory of Texas, which had declared its independence from Mexico in 1836. The United States annexed Texas in 1845, and Mexico refused to recognize the annexation. In April 1846, the United States declared war on Mexico, and the war lasted until February 1848. The United States emerged victorious, and the Treaty of Guadalupe Hidalgo was signed in February 1848, which ended the war and ceded much of Mexico's northern territory to the United States.
